1. Field of the Invention
The present invention relates to a method for preparing a semiconductor, which will find its application in the electronic or optoelectronic industry.
More particularly, the invention relates to a method for preparing a III-V semiconductor of general chemical formula AlxGa1-xN, which will find its use in diodes.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
The production of light-emitting diodes (LED) is currently in full swing and numerous innovations have been brought since the appearance of diodes in the sixties. Indeed, quasi exponential growth, according to Craford's law, has been observed in luminous efficiency and has enabled to increase the value which was smaller than 0.2 Im/V during the appearance on the market up to more than 20 lm/W today, thus exceeding the efficiency of non-filtered filament lamps.
The technology supporting LED diodes rests with the properties of the semiconductors. Typically, an LED is formed of a semiconductor having regions with different conductivity: a first region abundant in electrons, so-called n-type, and a second one abundant in electronic gaps, so-called p-type. Most recent LEDs, although operating according to the same principle, may combine different semiconductors (heterojunction LED), wherein the general principle remains however the same. The movement of the load carriers, commonly named “carrier”, electrons or holes, within the LED enables by re-combination to generate photons.
The luminous properties of a semiconducting material often rest with the defects and the impurities present within it. The use of dopants enables to bring particular quality to the material whereof they modify the composition; thus it is, in particular, possible to enrich the material with foreign matters so as to confer increased n- or p-character thereto.
Doping is however not easy and remains associated with different factors. It is recognized in particular that, for the family of the nitride semiconductors, and contrary to the n-type doping, the p-type doping is often difficult to carry out.
The solubility of the dopant, which corresponds to the maximum concentration that an impurity may reach in a thermodynamically balanced material, is highly linked with temperature and remains a significant limiting factor. The site occupied by the dopant within the material is not negligible and impacts in particular its energy as well as the electric properties of the material.
The occurrence of defects, such as gaps, self-interstitial atoms or antisites in the structure of the material increased complexity of the positioning possibilities for the dopant and concomitantly the variations in properties. Finally, the ionizing energy of the dopant, which enables determining the fraction of dopant which may play a carriers part, is, for its own as well, linked with temperature and impacts the properties quite considerably.
Nitrogen-containing III-V-type semiconductors have been studied extensively and seem to be best suited for light emission at green, blue and ultraviolet wavelengths (UV) which were not available at solid light emitters.
P-type doping of gallium nitride alloys GaN, aluminum nitride AlN and aluminum-gallium nitride AlxGa1-xN, when x varies between 0 and 1, is rather difficult due to the fact that the potential of the dopants used are highly accepting (Fischer and al-1995): Mg (220-250 meV), Zn (340 meV), Cd (550 meV), C (230 meV). This causes very low activation rate and, consequently, relatively low doping level (less than 1018 cm3.
For instance, magnesium Mg, which is used universally for p-type doping of GaN and AlxGa1-xN alloys, is situated 220 meV above the valency band of GaN, which leads to a typical 1%-activation rate at room temperature.
Theoretical calculations have shown that beryllium Be could be an efficient acceptor, with an activation energy around 60 meV (Bemardini and al 1997), Still, certain experiments have shown that the linking energy of Be is rather 240 meV (Nakano and al-2003), which makes Be as little interesting an element as a potential dopant.
The situation is even more dramatic in the case of AlGaN since it is known that the energy of certain acceptors, in particular Mg, becomes deeper and deeper in alloys with increasing Al content, turning these alloys into compounds which are difficult to be doped. This leads naturally to a limitation of their usage for the production of UV-type light-emitting diodes.
An approach to overcome the difficulties associated with p-type doping consists in co-doping Mg with oxygen 0, zinc Zn or with any other element, as described, for instance, by Korotkov et al 2001, Kipshidze et al 2002, Kim et al 2000) or still by Van de Waite et al 2004.
Even if a diminution in linking energy has been observed effectively when Mg was co-doped with 0, in practice this has not enabled to reach the doping levels desired.
Besides, attention should be drawn to the fact that co-doping is difficult to control since the ratio between the different codopants must be precise. For instance, it can be reminded that oxygen is a donor in GaN and that insufficient control in co-doping and oxygen content may lead very easily to undesirable n-type doping of the product.
Considering what was described above, it appears that efficient p-type doping of the GaN, AlN and AlxGa1-xN alloys is on the agenda.
The aim of the present invention is to provide a method for preparing a III-V semiconductor which remedies the shortcomings aforementioned, in particular as regards their doping.
Another aim of the present invention is to provide in particular a p-type doping method of the semiconductors of the AlxGa1-xN family.
Another aim of the present invention is to offer semiconductors capable of being used in the production of diodes, in particular light-emitting diodes in the UV ultraviolet zone.
Another aim of the present invention is to offer semiconductors capable of being used in the electronic or optoelectric industry.
Other aims and advantages of the invention will appear in the following description, which is given only by way of example and without being limited thereto.